You can use liquid nitrogen as a fuel for an engine. Here is how it works:
Normally, petroleum (like gasoline) is used as a fuel because it is liquid at normal temperature, and when you ignite it, it turns into a hot 500 degree Farenheit high-pressure gas and expands. You can harness the expansion to push a piston which turns the wheels of your car or propeller. Meanwhile, the heat you generate must be radiated into the atmosphere.
Now, take nitrogen. Nitrogen is a liquid at very low temperature (-417 degrees Farenheit). At normal temperature it is a gas. So, fill your tank with liquid nitrogen. Then let it into your engine, where the heat from the the outside air turns the liquid nitrogen into a high-pressure gas and expands. You can harness the expansion to push a piston, just like you harness the expansion of high-pressure burning
gasoline. Meanwhile, the cold engine must draw in heat from the atmosphere to warm it back up.
See http://students.washington.edu/rhein/Article.html
This idea is very clever indeed.
The principle of any engine is that you must have one side that is hot and the other side is cold. You convert some of the heat that flows from the hot side to the cold side into useful work.
Usually, the side that is hot is made hot by burning gas, and the side that is cold is normal air temperature. But using liquid nitrogen as a fuel means that the side that is hot is the normal air temperature, and the side that is cold is the -400 F liquid nitrogen.
Be aware it still takes energy to turn gaseous nitrogen into liquid nitrogen, so you are not getting something for nothing!

I don't think liquid nitrogen can be used as a "fuel," but it can be used to create thrust.


In an jet aircraft or rocket, when burning fuel, a hot gas is created which is then expelled out of the engine.Because this gas has a net momentum (mass times velocity) in one direction, the aircraft must move in the other direction to conserve momentum.


Another possibility, however, is to have a big tank of pressurized
gas (nitrogen) which you expel in a controlled manner. Think about what happens when you blow up a balloon and let go. The gas shoots out of the balloon in one direction and the balloon goes in the other direction.


You can do the same thing with liquid nitrogen. Storing nitrogen as a liquid is a much more compact way to store a lot of nitrogen.
Just boil off a little liquid nitrogen and you get a lot of gas.


You have to be careful, though, because the pressure can build up very rapidly as the nitrogen boils off. The tank can actually explode if there is no way built in to relieve excess pressure. How high of a pressure do think will cause a metal tank to blow up?

Your question is a good one, since the use of liquid nitrogen as rocket fuel has been making news lately. Check out this entry from the latest
issue of "mini-AIR", an e-mail supplement to the journal "Annals of Improbable Research":
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2001-05-05 A Chemical Punch
There has been a novel advance in the chemistry of propellants.
Investigator Keith G. Tomazi reports:
"I just came across a novel rocket fuel, as reported on Page 1 of the Wall Street Journal (Tuesday, April 10, 2001):
The New York City-based Fighters' Institute for Survival and Training, or FIST, helps boxers with life outside the ring.
But the group...hopes to become a union for boxers....
That pace sounds familiar to Paul Johnson, a Minneapolis ex-boxer. He has tried to get the Boxer's Organizing
Committee off the ground for about 12 years....Still, he believes that a union is imminent. "We've got the rocket ship on the pad, we're pumping liquid nitrogen, and we're going to blast off," he says.
"This is one picket line that I wouldn't care to cross. Still, their chemistry seems unusual: I wonder if anybody else has proposed a non-flammable rocket fuel?"
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Rocket fuels currently in use (liquid hydrogen and oxygen, eg) are flammable: if you ignite them, they burn. The combustion process is what propels the rocket into space. The conversion of liquid or solid fuel into a gas during combustion causes a large increase in air pressure; when the gas is forced out the back of the rocket at high enough speeds (sometimes
16,000 km per hr!), the force will propel the rocket forward. As mentioned above, liquid nitrogen is not flammable. It won't burn. Liquid nitrogen does build up a lot of pressure if kept contained inside a tank, however,
because at temperatures above -320 F, nitrogen will boil (convert from a liquid to a gas). This change in physical state causes a large increase in air pressure. The pressure generated from heating liquid nitrogen inside an incompressible chamber can be used to propel homemade rockets and even cars but cannot be used to propel aerospace rockets. Current research is looking into ways to make rocket fuel lighter, compact, and more powerful. This would increase the payload (cargo and people) that rockets could carry and
reduce their travel time, making large-scale space stations possible. With some fuels currently in development, NASA scientists estimate that we could make the trip to Mars in two weeks.
A good web site: http://www.channel4.com/plus/cosmo/rocket_science.html